OP, you might throw in a short line that you're only talking about the modern generator hubs of the last decade or so. Vintage Dynohubs produce different voltages and/or wattages than today's offerings.
Bit of trivia for you that may be of interest.
Whilst I currently use a Son28 hub to power my lights, gps and phone my first dynohub was one of the vintage Sturmey Archer GH6 models.
When I became aware of the new led headlights a couple of years ago, I ordered a Son hub and Edelux headlight.
This initially tapped me out financially for a short period of time and I couldnt afford to get my Son hub made up into a wheel.
I decided to try to see if my Sturmey GH6 would in any way wake up the Edelux headlight.
I didnt have much faith as the GH6 was reputed to be around 1.8 watts.
Well happy to report it worked fine.
So well in fact that if I hadnt already bought the Son dynamo hub, I'd likely not have bothered.
I'm glad I did, not so much because its likely the Son offers more output or perhaps less drag, but because the GH6 weighs in at around 1.1kg from memory, which is more than double the Son.
My bike was already heavy so swapping out the hub and sprung leather saddle for a Ti framed Brooks Swift meant a large weight saving.
I should probably add my bike at the time was a Moulton APB which utilises 20' wheels, meaning the hub was rotating faster than a larger diameter wheel of say 700c and thus putting out more power than the 700c or 26' would have.